For years my Fallout 3 strategy with regards to Vault 92 has always been the same: wait for an alien ship to crash during a random encounter, fetch the Alien Blaster and (with Dogmeat's help) all its ammo, then use ONLY on Deathclaws. And when it comes to Fallout 4, I suspect it will be as good a strategy now as it was then, especially with the addition of Alpha Deathclaws, an introduction made in Fallout: New Vegas. Here's how you get yours out in the Commonwealth.
This weapon can be found through a random encounter at Forest Grove Marsh, a flooded neighborhood filled with Ghouls south-southeast of Sanctuary Hills, about midway through the map. However, if you fail to follow the clues from that encounter, you can instead fast travel to your settlement of Oberland Station and travel a short path east to find a crashed alien ship. Follow the trail of splattered green blood to a nearby cave (the entrance will show up on your local map). The Alien Blaster Pistol is located on the floor. Be sure to kill the alien and loot its body for all the ammo.
